1. Field of the Invention
The present invention relates to the manufacture of a color cathode ray tube, and more particularly to a manufacturing method of a flat-face type color cathode ray tube having a black matrix film which is constituted of phosphor pixels and a light absorbing substance layer surrounding the phosphor pixels on an inner surface of a panel portion thereof.
2. Description of the Related Art
A color cathode ray tube, for example, a color cathode ray tube which is used in a color television set, a color display monitor for an OA (Office Automation) equipment terminal includes a vacuum envelope. The vacuum envelope is constituted of an approximately rectangular panel portion which has a phosphor screen including a black matrix (BM) film or a large number of dot-like or stripe-like phosphor pixels on an inner surface thereof, an approximately cylindrical-shape neck portion which houses an electron gun therein, and an approximately funnel-like funnel portion which connects the neck portion and the above-mentioned panel portion on an axis which is substantially coaxial with a tube axis and includes a deflection yoke on an outer periphery of a transitional region between the neck portion and the panel portion. Further, in the inside of the vacuum envelope, a shadow mask which constitutes a color selection electrode and includes a large number of electron beam passing apertures is arranged in the vicinity of the phosphor screen in an opposed manner.
The above-mentioned shadow mask uses an aluminum killed steel as a main constituting material thereof Further, with respect to the shadow mask, along with a recent demand for high definition of the color cathode ray tube, a shadow mask having a small plate thickness has been used. In a color cathode ray tube which adopts the small-plate-thickness shadow mask, a phenomenon in which a portion of the shadow mask is deformed by heat so that an electron beam spot is displaced from a given position on a phosphor surface during a displaying operation, that is, a so-called mask doming phenomenon is liable to easily occur. As a means to cope with such a phenomenon, along with the improvement of a shadow mask suspension mechanism, a material of INVAR, which is a material of FeNi36, is also used as the constitutional material in view of the thermal expansion coefficient and the physical hardness.
Such a shadow mask is formed as follows. A form in which a large number of electron beam passing apertures are formed at given positions by etching is blanked in a given shape. Thereafter, the blanked form is formed into a shape using a press such that the shadow mask is constituted of an approximately spherical main surface and a skirt portion which is contiguously formed with a periphery of the main surface and is bent by approximately 90 degrees with respect to the main surface and is used.
Further, recently, along with the popularization of a color television set or a color display monitor having a flat screen type, there is observed a tendency that an outer surface of a faceplate (panel glass) is leveled or flattened with respect to the color cathode ray tube which is used in the color television set and the color display monitor.
FIG. 11 is a schematic cross-sectional view for explaining a constitutional example of a shadow-mask-type color cathode ray tube of a flat face type. In FIG. 11, a vacuum envelope is constituted of a panel portion 51 which forms a phosphor screen 50 having a black matrix film which consists of phosphor pixels and a non-light-emitting light absorbing material layer on an inner surface thereof, a neck portion 52 which houses an electron gun 61, and a funnel portion 53 which connects the panel portion 51 and the neck portion 52.
The panel portion 51 includes an approximately flat outer surface and a concavely curved inner surface. The phosphor screen 50 which is arranged on the inner surface of the panel portion 51 includes, in general, phosphor pixels which are formed by applying phosphors of three colors of red (R), green (G), blue (B) respectively in a dotted pattern or in a stripe pattern, a black matrix film which surrounds the phosphor pixels and is formed of a non-light-emitting light absorption material layer made of carbon, and a metal reflection film which constitutes a metal back layer. Further, a shadow mask 54 is arranged close to the phosphor screen 50. The shadow mask 54 is formed of INVAR material by taking a thermal expansion coefficient and a physical hardness into consideration.
The shadow mask 54 is of a self-standing shape-holding type which is formed by a press, wherein a periphery of the shadow mask 54 is welded to a mask frame 57, and the shadow mask 54 is suspended and supported on stud pins 60 which are mounted upright on an inner wall of a skirt portion of the panel portion 51 by way of suspension springs 59. Here, a magnetic shield 58 is fixed to an electron-gun-61-side of the mask frame 57. A deflection yoke 55 is exteriorly mounted on a transitional region between the neck portion 52 and the funnel portion 53 of the vacuum envelope, wherein by deflecting three modified electron beams B which are irradiated from the electron gun 61 in the horizontal direction (X direction) and the vertical direction (Y direction), the electron beams B are scanned two-dimensionally on the phosphor screen 50 thus reproducing the image.
Further, an inner conductive film 62 which is formed on an inner surface of the funnel portion 53 applies a high voltage introduced from an anode button to electrodes which form a main lens of an electron gun 61 and a metal reflection film of the phosphor screen 50. Numeral 63 indicates a reinforcing band, numeral 64 indicates a mouthpiece, and numeral 65 indicates a whole color cathode ray tube.
In the color cathode ray tube having such a constitution, as described previously, the panel portion 51 has the approximately flat outer surface and the concavely curved inner surface. To the contrary, the shadow mask 54 is shaped into the given curved surface by molding the shadow mask form using a press and is curved in conformity with the inner surface of the panel portion 51.
The reason the inner surface of the panel portion 51 and the shadow mask 54 are curved irrespective of the approximately flat external surface of the panel portion 51 is that the manufacturing method of the shadow mask 54 by a press forming technique can be performed easily and at a low cost.
The curved shape of the shadow mask 54 is an aspherical shape in which radii of curvature are gradually decreased from the center of a main surface to a periphery of the shadow mask 54 respectively along a long axis, a short axis and a diagonal line of the shadow mask 54 respectively. The curvatures of the shadow mask 54 of the aspherical shape are determined as follows, for example, wherein an equivalent radius of curvature is set as Re. That is:Re=(z2+e2)/2z
Here, e: a distance (mm) in the direction orthogonal to a tube axis from the center to an arbitrary peripheral position on a main surface of the shadow mask
z: a falling quantity (mm) in the tube axis direction from the center of the main surface of the shadow mask at the above-mentioned arbitrary peripheral position
Such specification establishes the compatibility between a flat feeling of the screen and the maintenance of a mechanical strength of the shaped shadow mask in the color cathode ray tube
FIG. 12 is a schematic cross-sectional view showing a portion of an essential part of the color cathode ray tube shown in FIG. 11 in an enlarged manner. In FIG. 12, The phosphor screen 50 formed on the inner surface of the panel portion 51 includes three-color phosphor pixels 501 which are formed by applying phosphors of three colors in a dotted pattern or a stripe pattern, a black matrix film 502 which surrounds the phosphor pixels 501, and a metal reflection film 503, wherein the shadow mask 54 is arranged close to the phosphor screen 50 in a state that the shadow mask 54 faces the phosphor screen 50 in an opposed manner.
The three-color phosphor pixels 501 are constituted of a red (R) phosphor pixel 501R, a green (G) phosphor pixel 501G and a blue (B) phosphor pixel 501B. The phosphor pixels 501 are formed on opening portions (window portions) formed in the black matrix film 502 through an exposure step after applying a phosphor slurry on an inner surface of the panel portion on which the black matrix film 502 is formed. The exposure step is performed for every color. Since positions of three light sources 66G, 66B, 66R are different from each other, it is possible to accurately form three kinds of phosphor pixels on the opening portions (window portions) formed in the black matrix film 502 respectively.
One example of a conventional exposure device which is used for forming such a phosphor screen is shown in FIG. 13. The exposure device shown in FIG. 13 is an exposure device which is disclosed in FIG. 1 of patent document 1, that is, patent publication number JP-A-8-185798. In FIG. 13, numeral 1 indicates a spot light source, numeral 2 indicates a correction lens, numeral 3 indicates a light control filter, numeral 4 indicates a shadow mask, numeral 5 indicates a panel, numeral 6 indicates guide poles, numeral 7 indicates an XYθ table, numeral 8 indicates a position adjusting mechanism, and numeral 9 indicates a light control filter mounting plate. In forming respective BMs for blue, red and green using such an exposure device, the exposure is performed by displacing the position of the spot light source 1. Accordingly, the exposure is performed by preliminarily moving and setting the light control filter 3 to a position where the difference among the respective BM diameters becomes optimum. Patent document 1 discloses that since the BMs for three primary colors are formed using the same light control filter 3 in the above-mentioned manner, it is possible to prevent the generation of BM diameter difference attributed to the irregularities of the light control filter 3 as a finished product.
Further, FIG. 14 is a schematic view showing a conventional exposure method for forming a phosphor screen and is disclosed in FIG. 1 of patent document 2, that is, patent publication number JP-A-7-122184. In FIG. 14, light from an exposure light source 36 passes a group of lenses 35, the light which passes the group of lenses 35 is irradiated to an inner surface of a panel glass 32 of a cathode ray tube, thus performing the exposure for forming a phosphor screen 34 of the cathode ray tube. Here, the light transmissivity of a peripheral region of at least one lens or filter which is included in the group of lenses 35 is set higher than the light transmissivity of a center region of the lens or the filter thus making the intensity distribution of exposure light on the peripheral region with respect to the center region of the panel portion appropriate.
Patent document 2 discloses that with the use of such method and exposure device, even on an inner surface of a panel glass which is used in a cathode ray tube having an ultra wide angle, it is possible to form a phosphor screen pattern at an accurate width and pitch.
In the flat panel type color cathode ray tube shown in FIG. 11 in which the panel portion has the approximately flat outer surface, a wall thickness of the panel portion differs between a center portion and a peripheral portion. Due to such difference in thickness, there has been a drawback that the difference arises among the brightness of three colors thus giving rise to a possibility that the white uniformity is deteriorated. To cope with such a drawback, it is necessary to correct only a BM width of one color.
In such a case, it is necessary to prepare grading filters which differ in transmissivity. However, as described in patent documents 1 and 2, the grading filter such as the light control filter 3 or a CAD (Computer Aided Design) filter 42 has the specification common in three colors and hence, it is difficult to exchange the grading filter in the midst of the exposure. As a compensating means to overcome such difficulty, the exposure may be performed by combining the grading filter and a local adjusting filter so as to adjust the above-mentioned BM width. However, when the above-mentioned local adjusting filter is added, due to the mutual interference of both filters, it is hardly possible to obtain a desired phosphor screen.